Globular structures in roots accumulate phosphorus to extremely high concentrations following phosphorus addition.

Crops with improved uptake of fertiliser phosphorus (P) would reduce P losses and confer environmental benefits. We examined how P-sufficient six-week-old soil-grown Trifolium subterraneum plants, and two-week-old seedlings in solution culture, accumulated P in roots after inorganic P (Pi) addition. In contrast to our expectation that vacuoles would accumulate excess P, after 7 d X-ray microanalysis showed that vacuolar [P] remained low (<12 mmol kg-1 ). However, in the plants after P addition some cortex cells contained globular structures extraordinarily rich in P (often >3000 mmol kg-1 ), potassium, magnesium and sodium. Similar structures were evident in seedlings, both before and after P addition, with their [P] increasing three-fold after P addition. NMR spectroscopy showed seedling roots accumulated Pi following P addition, while TEM revealed large plastids. For seedlings we demonstrated that roots differentially expressed genes after P addition using RNAseq mapped to the T. subterraneum reference genome assembly and transcriptome profiles. Amongst the most up-regulated genes after 4 hr was TSub_g9430.t1, which is similar to plastid envelope Pi transporters (PHT4;1, PHT4;4): expression of vacuolar Pi-transporter homologs did not change. We suggest that subcellular P accumulation in globular structures, which may include plastids, aids cytosolic Pi homeostasis under high-P availability.